Polyester blends with improved stress whitening for film and sheet applications

Abstract

Disclosed are polyester blends that provide improved stress whitening behavior. The blends may be prepared from a polyester having crystallization half-time of at least 5 minutes and a polyester elastomer. The polyesters blends may be extruded or, in the presence of a release additive, calendered to produce film or sheet. Also disclosed are processes for making a film or sheet by extrusion or calendering the above blends and the film or sheet produced therefrom. The polyester blend and the film or sheet produced therefrom also may include plasticizers and / or flame retardants to increase their flexibility and enable their use in commercial applications requiring flame resistance. The film and sheet have excellent appearance and can be used in a wide range of decorative and packaging applications.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Patent Application No. 60 / 627,754, filed on Nov. 12, 2004; the entire content of which is hereby incorporated by reference.FIELD OF THE INVENTION [0002] This invention pertains to polyester blends for extrusion and / or calendering and, more specifically, to polyester blends which enable improved stress whitening behavior. The invention further pertains to extrusion and / or calendering processes for these polyester blends and to polyester film or sheet produced therefrom. BACKGROUND OF THE INVENTION [0003] Film and sheet products are often stressed and cold formed during the assembly process in many applications. For example, in some appliance applications, opaque and colored films are often laminated to metal in place of paint. In this case, the polymer film is laminated to the metal in a flat form. The metal with the polymer film lamination is then bent or pressed into shape to form...

AI Technical Summary

Benefits of technology

[0012] (c) optionally, a release additive in an amount effective to prevent sticking of the polyester blend to calendering rolls.

[0013] The polyester blend of the present invention may be calendered or extruded to produce a film or sheet having excellent stress whitening properties, good surface properties, toughness, and flexibility. The blend of the invention may include other additives such as, for example, plasticizers, flame retardants, antioxidants, antiblocking agents, fillers, colorants, and pigments. The polyester may be any amorphous or substantially amorphous polyester having a crystallization half-time from the molten state of at least 5 minutes. The polyester typically will comprise at least 80 mole percent of the residues of terephthalic acid based on the total moles of diacid residues. The polyester elastomer may comprise any random or block copolyester that exhibits elastomeric properties. Representative examples of polyester elastomers include, but are not limited to, random or block poly(ether ester) polymers comprising polyester segments and polyether segments having molecular weights of 400 to 12,000, and aromatic-aliphatic polyesters.

[0021] (c) optionally, a release additive in an amount effective to prevent sticking of the polyester blend to calendering rolls.

[0027] (c) optionally, a release additive in an amount effective to prevent sticking of the polyester blend to calendering rolls.

[0030] The polyester blends, films, and sheets of the invention may also include plasticizers and / or flame retardants to increase their flexibility and enable their use in commercial applications requiring flame resistance. The films or sheets have an excellent appearance, flexibility, and flame retardancy and can be used in a great variety of decorative and packaging applications. The films or sheets are readily thermoformed into various shapes for specific packaging applications for both food and non-food products. They may be printed with a wide variety of inks and may be laminated either in-line or off-line with fabrics or other plastic film or sheet. Some specific end uses include graphic arts, transaction cards, security cards, veneers, wall coverings, book bindings, folders and the like.

Problems solved by technology

During this cold forming process, some polymer film formulations tend to whiten as they are bent and stretched into shape and cause unattractive white streaks to appear in the finish of the appliance.

This problem is more pronounced with darker colors.

For example, PET polymers with inherent viscosity values of about 0.6 deciliters / gram (abbreviated herein as “dL / g”), typically have insufficient melt strength to perform properly on the calendering rolls.

PET and other polyester polymers also are prone to crystallize at typical processing temperatures of 160° C. to 180° C., resulting in a non-homogeneous mass which also causes high forces on the calender bearings.

Higher temperatures, however, can cause degradation of the polyester such as, for example, by thermal degradation, hydrolysis of polymer by exposure to atmospheric moisture, and the formation of color bodies.

Typical PET polymers also have a tendency to stick to the calendering rolls at higher processing temperatures.

Although some of these difficulties can be avoided by the careful selection of polymer properties, additives, and processing conditions, calendering of polyesters at high rates of production is difficult.